I think what has happened is that the notebook runs on mains power (no battery inserted) and was unplugged before windows shut down properly (maybe installing updates). I'm not 100% sure of this but it is what I suspect.

The BIOS battery is a backup for desktop computers, used only when the mains power does not enter the PSU. With laptops, it is in use more frequently, and may last only two years or so. Once the battery is replaced, you have to reset all the BIOS pages to reflect the true system setup.
That "CMOS checksum error" message is a general warning that something is mismatched between BIOS settings and actual hardware. Could be any one of more than 20 features.